Production of non-corrosive naphthas



United States Pfltfiflt O F PRODUCTION OF NON-CDRROSIVE NAPHTHAS William L. Jacobs, Crystal Lake, and Weldon G. Annable, Muntlelein, 111., assignors to The Pure Oil Company, Chicago, lib, a corporation of Ohio No Drawing. Application August 2, 1955 Serial No. 526,054

12 Claims. (Cl. 196-28) This invention is directed to a method for the production of petroleum naphthas characterized by their ability to pass the distillation-corrosion test. More particularly, the invention relates to the production of noncorrosive hydrocarbons, which have been otherwise desulfurized and sweetened, by washing with a two-step treatment comprising washing with a solution of sodium polysulfide, followed by washing with a relatively strong caustic solutlon.

Crude petroleum has long been the source of widely known products including gasoline, kerosene, diesel fuels, lubricating oils, and heavy tars. In many instances, the products obtained from petroleum are employed as reactants in the synthesis of additional petroleum derivatives and chemicals and a large number of products of petroleum are used directly without extended treatment or modification. Petroleum naphthas comprise a wide variety of such latter products and are used extensively in the dyeing, rubber extraction, protective coating, and allied industries. A large portion of the petroleum naphthas used in straight-run naphthas which are selected fractions of the lower boiling, more volatile constituents of crude petroleum. The present invention is directed to a method of transforming deleterious sulfur compounds present in hydrocarbon mixtures into forms which are less obnoxious and non-corrosive, and will be illustrated by the treatment of straight-run naphthas. The examples given are not to be construed as limiting the invention. The term naphthas as herein used shall mean straightrun petroleum naphthas and other hydrocarbon mixtures or their equivalents containing deleterious sulfur compounds which must be transformed to meet rigid corrosion tests.

Naphthas prepared from petroleum by physical means inevitably contain other types of organic and inorganic compounds due to the complex nature of petroleum, which are deleterious as far as certain end uses of the naphthas are concerned and necessitate the application of additional refining steps. Even with such additional refining, it is exceedingly difiicult to prepare naphthas which meet the exacting specifications that have been established by the industry. Of these deleterious nonhydrocarbon compounds, the sulfur and sulfur-containing constituents are generally the most persistent and cling tenaciously to any environment in which they exist, imparting objectionable odor, corrosiveness, color, and other physical and chemical properties thereto. The odor of naphthas is important; however, no standard test exists to cover this property and the odor of a well-refined naphtha is generally described as sweet.

Tests have been devised to determine both quantitatively and qualitatively the presence of these odious compounds in an attempt to control the properties and quality of naphthas from petroleum sources. For this purpose, various copper strip corrosion tests, the mercury test, the lead acetate test, and the doctor test are used. Procedures established by A. S T. M. are used to deter mine the content and distribution of these sulfur com- 2,843,528 Patented July 15, 1958 pounds. Perhaps the most critical and rigorous qualitative test for determining the presence of corrosive sulfur compounds in naphthas is the distillation-corrosion test, known also as the Philadelphia test, the Amsco corrosion test, or the full boiling range corrosion test-by any name, a particularly rigorous species of copper strip corrosion test. The test, widely applied by the manufacturers, distributors, and users of specialty naphthas, is carried out by the addition of a small pure copper coupon to an ordinary A. S. T. M. distillation flask containing cc. of the naphtha to be tested. The copper strip is so positioned in the flask that one end of the strip contacts the residue at the end of the distillation, and the distillation is conducted according to A. S. T. M. D 86-38 as described in A. S. T. M. Standards on Petroleum Products and Lubricants, published by the American Society for Testing Materials, Philadelphia, Pennsylvania.

At the completion of the test, wherein the fllask has been heated to dryness, the color of the copper strip is an indication of the relative amount of corrosive sulfur compounds present in the naphtha sample. A negative test is shown by the presence of a very slight or moderate tarnish on the strip and stamps the naphtha as satisfactory. If the copper strip becomes moderately blackened, the results are interpreted as positive or unsatisfactory. The production of a slightly tarnished or slightly colored or corroded strip, indicated by a dark orange with peacock colorations thereon, is termed borderline and as such denotes a naphtha which is not acceptable and must be further refined. The market is limited for offspecification naphthas and further refining is expensive since even then there is no assurance that the product will pass the severe distillation-corrosion test.

The subjection of high sulfur content naphthas to various refining and sweetening operations which may include oxidation and extraction methods, or the recycling of rejected off-specification naphthas back through such a process, does not produce acceptable naphthas because the sulfur compounds remaining are the most difficult to remove and the most corrosive. High sulfur content naphthas usually have a poor odor as well as other undesirable properties. If straight-run naphthas from high sulfur crudes are subjected to other more severe refining methods, the resulting products may pass the other tests for sulfur compounds but do not pass the distillation-corrosion test. Often naphthas are produced which are negataive or borderline to the distillationcorrosion test and which exhibit a positive reaction to one or more of the other tests for sulfur compounds. Since naphthas must pass all such tests to be acceptable, further treatment is necessary. Prior art methods of desulfurization when applied to such naphthas may produce a doctor negative or mercury negative product, but in so doing the end result is a positive distillation-corrosion test.

Accordingly, the primary object of this invention is to overcome this problem and provide aprocess for produc ing improved naphthas by a two-step extraction method.

A second object of the invention is to provide a method of producing naphthas which pass the critical distillationcorrosion test from naphthas containing unacceptable amounts of sulfur compounds.

These and other objects of the invention will become apparent as the description thereof proceeds.

In the prior art there is extensive discussion and use of alkali solutions and sodium polysulfide solutions for the purpose of sweetening or removing odorous substances, such as mercaptans, from hydrocarbons. It is well-known that the said alkali polysulfides quickly and completely sweeten mercaptan-bearing hydrocarbon oil vapors and gases. The sweetening or oxidizing reaction isexhibited by the higher polysulfides of the alkali and alkaline earth metals and, more particularly, by the higher polysulfides of sodium, potassium and ammonia. It is also disclosed in the art that elemental sulfur can be removed from hydrocarbon oils by treatment with sodium, potassium or ammonium polysulfides. Reference was made to the combined use of polysulfides and alkalis for this purpose.

In accordance with the present invention, it has been found that these conventional methods of treatment are not. satisfactory for the removal of corrosive sulfur and/or sulfur compounds from naphthas or other hydrocarbon mixtures which have been previously desulfurized or otherwise sweetened; for example, naphtha which has been desulfurized at 600 to 800 F. over a cobalt molybdate catalyst with or without the presence of hydrogen is substantially free of mercaptans and other deleterious sulfur compounds but is found to contain a sufficient amount of other sulfur compounds or corrosive agents which deleteriously affect the naphthas and cause a positive distillation-corrosion test. It has been found that such materials can be transformed into acceptable products by solvent extraction or treatment with a sodium polysulfide solution, followed by a strong caustic wash. It has further been found that neither of these treatments alone will give a non-corrosive product and only this particular sequence of steps will apply. The treatment may or may not be followed by a water wash step.

In order to demonstrate the invention, different samples of the same naphthas having the following characteristics, ASTM boiling range of 160-380" F. and a gravity of 55.5 API, were treated in accordance with the pro cedure set forth in Table I. The naphtha was a hydrodesulfurized straight-run naphtha. The sulfur content of this material was less than about 0.003 wt. percent. The naphtha contained a slight trace of hydrogen sulfide from hydrodesulfurization over cobalt molybdate catalyst, and consequently the naphtha was positive to the doctor test. After caustic washing, the naphtha was negative to the doctor test but failed in the distillation-corrosion test indicating the presence of free sulfur or other corrosive materials.

The sodium polysulfide solution used for treating the naphtha was prepared by dissolving 156 gm. of commercial sodium sulfide in a minimum amount of water and diluting to 1 liter. 18 gm. of elementary sulfur and 40 gm. of sodium hydroxide were then added and the solution thoroughly agitated. Both treating solutions were used in the ratio of 1 volume of solution to 3 volumes of naphtha.

From the table it is seen that a caustic wash alone or a sodium polysulfide wash alone does not produce a product which passes the distillation-corrosion test. Furthermore, the use of two or more applications of sodium polysulfide or two or more applications of caustic also fails to produce the desired result. It is only when the sodium polysulfide wash is followed by a caustic wash with or without a final water washing that a product passing the distillation-corrosion test is obtained.

The caustic solution used in the washing or extraction step may vary in concentration from about 25 wt. percent to 50 wt. percent. The use of caustic solutions having below about 25 wt. percent is not recommended because the desired results are not insured and excessively large quantities of caustic solution may be required, thus in-' creasing the difiiculty in obtaining adequate contact.

Although sodium hydroxide has been used to demonstrate the invention, other alkali metal hydroxides such as potassium hydroxide may be used. The temperature of contact may vary from room temperature to as high as the initial boiling point of the naphtha. No enhanced results are obtained by the use of higher or lower temperatures or vapor phase treatment. The sodium polysulfide solution may contain from about 5 to 20 wt. percent of sodium polysulfide, calculated as Na S Both treating solutions may be used in the ratio of 1 volume of solution to 2 to 4 volumes of naphtha, although a ratio of 1 to 3 is preferred.

The virgin naphtha fraction selected from the crude is first subjected to a catalytic hydrodesulfurization treatment carried out in accordance with well-known techniques at elevated temperatures. In the treatment, the sulfur content of the charge stock is removed in the form of a gas such as hydrogen sulfide by the action of hydrogen and desulfurization catalysts, such as molybdates, sulfides, and oxides of iron group metals and mixtures including cobalt molybdate, chromic oxide, vanadium oxide with molybdena and alumina, and sulfides of tungsten, chromium, or uranium. Preferred catalysts for the reaction include cobalt molybdate, cobalt oxide-molybdena-alumina, and chromia-molybdena-alumina. The process may be carried out in either the liquid or gaseous phase at temperatures ranging from 500 to 800 F. and under pressures from 20 to 1000 pounds per square inch. The virgin naphtha fraction subjected to hydrodesulfurization may contain from about 0.1 to 3.0 percent by weight of sulfur. The charge may be introduced to the catalyst zone at from 10.5 to 10 liquid volumes per bulk volume of catalyst per our.

The preferred conditions of hydrodesulfurization are at approximately 750 F. under 250 pounds per square inch pressure and with a space velocity of 0.3 to 2.0 with hydrogen recirculated at a rate of about 3000 s. c. f. of hydrogen per barrel of charge.

The products from the hydrodesulfurization are subjected to stabilization wherein the hydrogen sulfide and any fixed gases are removed and the resulting stabilized liquid products are next subjected to a mild, low-temperature dehydrogenation or hydroforming reaction. During the stabilization, hydrogen sulfide is removed from the liquid products. The removal of the hydrogen sulfide may also be accomplished by extraction with an amino solution. The hydrogen may be purified and recycled to the first stage of the process. The hydrogen sulfide removed may be used to prepare free sulfur. The sour product from the hydrodesulfurizatiou is then subjected to the two-step extraction process heretofore outlined.

What is claimed is:

1. The process for producing special solvent naphthas from petroleum hydrocarbon mixtures containing corrosive sulfur compounds which comprises separating a virgin naphtha fraction from said mixtures, subjecting said fraction to catalytic hydrodesulfurization under conditions such that hydrogen sulfide is produced, separating the hydrogen sulfide so formed to produce a hydrodesulfun'zed product, subjecting said hydrodesulfurized product to contact with an alkali metal polysulfide solution, separating the naphtha fraction from said solution, subjecting the naphtha fraction to contact with an alkali metal hydroxide solution and separating a naphtha product characterized by its ability to pass the distillation-corrosion test.

2. The process in accordance with claim 1 in which said petroleum hydrocarbon mixtures contain at least about 1.0 weight percent of total sulfur.

3. The process in accordance with claim 2 in which said catalytic hydrodesulfurization is conducted at a temperature of from 500 to 800 F. in the presence of a catalyst selected from the group consisting of cobalt molybdate, cobalt oxide-molybdena-alumina, and chromia-molybdena-alumina.

4. The process in accordance with claim 3 in which the hydrodesulfurized product has a sulfur content of less than about 0.003 weight percent.

5. The process in accordance with claim 4 in which the hydrodesulfurized product is treated with an alkali metal polysulfide solution comprising sodium polysulfide.

6. The process in accordance with claim 5 in which the naphtha product is treated with an alkali metal hydroxide solution comprising sodium hydroxide.

7. The process in accordance with claim 6 in which the sodium hydroxide solution has a concentration of between about 25 to 50 wt. percent.

8. The process in accordance with claim 7 in which about 2 to 4 volumes of said naphtha product are treated l with 1 volume of treating solution during each contacting step.

9. The process in accordance with claim 5 in which the sodium polysulfide solution has a concentration of between about 5 to 20 wt. percent.

10. The process for producing special solvent naphthas from petroleum naphthas containing about 0.003 wt. percent of total sulfur present as sulfur compounds which are corrosive to the distillation-corrosion test which comprises subjecting said naphthas to extraction with an alkali metal polysulfide solution, separating the naphtha from said solution, subjecting the naphtha to contact with an alkali metal hydroxide solution and separating a naphtha product characterized by its ability to pass the distillation-corrosion test.

11. The process in accordance with claim 10 in which the alkali metal polysulfide solution is sodium polysulfide having a concentration of between about 5 to 20 wt. percent.

12. The process in accordance with claim 10 in which the alkali metal hydroxide solution is sodium hydroxide having a concentration of between about 25 to wt. percent.

References Cited in the file of this patent UNITED STATES PATENTS 1,413,005 Cobb Apr. 18, 1922 2,020,661 Schulze et al. Nov. 12, 1935 2,150,149 Burk et a1. Mar. 14, 1939 2,273,299 Szayna Feb. 17, 1942 OTHER REFERENCES Vesselovsky et al.: Action of Alkali Hydroxides of Elementary Sulfur and Mercaptans Dissolved in Naphtha, Industrial and Engineering Chemistry, February 1931, vol. 23, No. 2, pages 182183. 

1. THE PROCESS FOR PRODUCING SPECIAL SOLVENT NAPHTHAS FROM PETROLEUM HYDROCARBON MIXTURES CONTAINING CORROSIVE SULFUR COMPOUNDS WHICH COMPRISES SEPARATING A VIRGIN NAPHTHA FRACTION FROM SAID MIXTURES, SUBJECTING SAID FRACTION TO CATALYTIC HYDRODESULFURIZATION UNDER CONDITIONS SUCH THAT HYDROGEN SULFIDE IS PRODUCED, SEPARATING THE HYDROGEN SULFIDE SO FORMED TO PRODUCE A HYDRODESULFURIZED PRODUCT, SUBJECTING SAID HYDRODESULFURIZED PRODUCT TO CONTACT WITH AN ALKALI METAL POLYSULFIDE SOLUTION, SEPARATING THE NAPHTHA FRACTION FROM SAID SOLUTION, SUBJECTING THE NAPHTHA FRACTION TO CONTACT WITH AN ALKALI METAL HYDROXIDE SOLUTION AND SEPARATING A NAPHTHA PRODUCT CHARACTERIZED BY ITS ABILITY TO PASS THE DISTILLATION-CORROSION TEST. 